The present invention concerns an air guide box for the drying section of a high-speed paper machine where the paper web to be dried meanders together with a backing belt forming a continuous backing belt loop across drying cylinders, and at that, alternately across drying cylinders that are located outside and inside the backing belt loop. An air guide box of the categorial type is known from the German patent document No. 32 36 576 which is equivalent to U.S. Pat. No. 4,502,231.
In a drying section using such air guide boxes, as is generally known, the drying cylinders are arranged in two tiered rows. The paper web and a continuous backing belt run alternately across cylinders of an upper and a lower row. The cylinders arranged in the one row, preferably the top row, are contained outside the continuous backing belt loop, whereas the cylinders of the other row, preferably the bottom row, are arranged within the continuous backing belt loop. The air guide box is always arranged on a backing belt section running from an outside (preferably top) to an inside (preferably bottom) cylinder.
The expression "cylinder" respectively "drying cylinder" is to be understood here quite generally, i.e., it covers not only steam-heated drying cylinders with a smooth cylinder surface but also guide rolls with a smooth, profiled or perforated surface which, when needed, may be fashioned as suction rolls. This is because the inventional air guide box is suitable for drying sections of various designs. In a preferable design, both the outside and inside cylinders are fashioned as steam-heated drying cylinders. In another preferred drying section design, only the outside cylinders are fashioned as steam-heating drying cylinders, whereas the inside cylinders, i.e., preferably those of the bottom row, are designed as guide rolls or as suction guide rolls.
Forces of various types act in the drying section of a high-speed paper machine on the paper web and the backing belt; as the paper web and the backing belt pass through the paper machine, these forces cause at increasing speed phenomena such as web flutter, web liftoff and wrinkling. These phenomena, which possibly may even lead to a break of the paper web, are to be avoided as much as possible.
This objective was extensively accomplished already with the air guide box according to the German patent document No. 32 36 576, which is known also by the name "web stabilizer". But specific examinations of the air flows, for one, and practical experience, for another, have shown that the prior air guide box can be optimized further in view of the requirements imposed on it.
In the practical operation of high-speed paper machines using air guide boxes according to the German patent document No. 32 36 576, the following was noticed at increasing machine speeds: There is a risk that the backing belt, which together with the paper web meanders across the drying cylinders, will at times brush against the air guide box. Such a contact occurs specifically on the so-called boundary layer stripper arranged on the entrance (preferably upper) end of the first box wall (also called "foil wall"). But an undesirable contact with the backing belt may occur also on the foil wall. Attempts were made at circumventing these problems by adjusting a greater space between the trajectory of the backing belt and the foil wall. But this approach was not satisfactory either because it invited the risk that--at least at times--the foil effect is lost, i.e., that an appreciable vacuum is no longer created in the gap between the backing belt and the air guide box.
Considered also was a replacement of the mechanical boundary layer stripper by a blowing air jet (air scraper) directed at the backing belt; refer to column 6, lines 14 and 15, of the German patent document No. 32 36 576. But this has the disadvantage that the consumption of blowing air increases considerably. Besides, there is the risk that the blowing air jet directed at the backing belt will penetrate the backing belt and cause the paper web to lift off the backing belt. But the objective is exactly the opposite, namely causing the paper web to safely adhere to the backing belt.
Previously known is an arrangement such that the entrance edge of the foil wall of the air guide box, on which the boundary layer stripper is mounted, is arranged in the area where the backing belt separates from the upper cylinder. This arrangement would result in the case of the prior air guide box when the vertical distance between the upper cylinder row and the lower cylinder row is smaller than illustrated in the German patent document No. 32 36 576. Such an arrangement, in principle, is known already in the case of a large vertical cylinder spacing. The length of the air guide box--viewed in cross-section--is in this case enlarged accordingly. But this increases also the risk of a contact between the backing belt and the foil wall of the air guide box and, thus, the risk of wear on these elements.
The problem underlying the present invention is therefore to so modify the design of the prior air guide box that the risk of wear is eliminated or at least reduced on the backing belt and on the air guide box itself, which is in no way to adversely affect the stability of the run of the paper web in contact with the backing belt, but rather to improve it further, thereby enabling in the future machine speeds that are higher yet than heretofore. This problem is solved by the present invention.
In other words, this means that the so-called foil wall of the air guide box is no longer of flat design such as before, but is adapted to the curved course of the backing belt that adjusts itself operationally. This adaptation is to be such that now as before (i.e., despite the curved course of the backing belt) the gap between the backing belt and the air guide box diverges gradually in the running direction of the backing belt, thereby ensuring the foil effect of the air guide box across its entire length, viewed in cross-section.
The inventional design takes into account that the course of the backing belt and the paper web deviate more or less from the theoretical, straight running course tangent to the two cylinder surfaces as the machine speed increases, with said course more or less bowing out toward the air guide box. The adaptation of the foil wall to this bow-out ensures that the backing belt and the foil wall will not make contact even at high machine speeds and that the entrance and edge of the foil wall--despite a large vertical cylinder spacing (and thus a great length of the air guide box--viewed in cross-section) --can be arranged at the point where the paper web and the backing belt are still in contact with the upper cylinder. In other words, the entrance end edge of the foil wall can be safely arranged in the area of the departure point of the backing belt from the upper cylinder and even before it (relative to the running direction). This latter variant will be explained below.
Thus, it is inventionally possible to always make use of the possibility of adjusting between the entrance end, on which now as before a boundary layer stripper is arranged, and the backing belt an exactly definable small spacing. This makes it possible for the boundary layer stripper to preferably work in noncontact fashion. This is true not only for the case when the boundary layer stripper is fashioned as a blowing air jet (air scraper), but it applies specifically also in the preferred case where a mechanical boundary layer stripper is concerned (felt strip, brush slat or similar). This eliminates also the risk that the seam that makes the backing belt continuous and is thicker than the backing belt itself will make contact with the boundary layer stripper. Additionally, there is now always the previously known advantage that the effect of the air guide box, namely the creation of a vacuum in the gap between the backing belt and the air guide box, begins already at the point where the paper web and the backing belt depart from the upper drying cylinder. This counteracts the tendency of the paper web to continue, at the departure point, clinging for a short distance to the cylinder surface of the drying cylinder and only then settle again on the backing belt.
As already indicated though, the boundary layer stripper need not be arranged exactly at the departure point of the backing belt from the upper drying cylinder. Rather, the invention makes it possible to extend the air guide box somewhat further upward so that the boundary layer stripper is situated within the so-called looping zone in which the paper web and the backing belt touch the curved cylinder surface of the upper drying cylinder. This is advantageous because stripping the boundary air layer carried by the backing belt is more effective in this area. This utilizes the phenomenon that a vacuum is created in the radially inner area of an air boundary layer proceeding along a curved course, so that the air in the radially inner partial layer is thinner than in the radially outer partial layers.